Material containing lignocellulose, such as wood or the like, is defibered in disc and conical refiners to produce different fibre pulps. Both the disc refiners and the conical refiners have two refiner discs with a refining surface on both of them. The disc refiners have a disc-like refiner disc and the conical refiners have a conical refiner disc. The refiner discs are mounted with their coaxially rotating refining surfaces against each other. Either one of the refiner discs then rotates relative to a fixed refiner disc, i.e. stator, or both discs rotate in opposite directions relative to each other. The refining surfaces of refiner discs typically have grooves and protrusions, or blade bars, between them, called bars in the following. The shape of these grooves and bars may vary in many different ways per se. Thus, the refining surface, for instance, may in the radial direction of the refiner disc be divided into two or more circular parts, with grooves and bars of different shapes in each of them. Similarly, the number and density of bars and grooves on each circle, and their shape and inclination may differ from each other. Thus, the bars may either be continuous along the entire radius of the refining surface or there may be several consecutive bars in the radial direction.
The refiner discs are formed in such a manner that the distance between the refining surfaces is longer in the centre of the refiner discs, and the gap between the refining surfaces, i.e. refining zone, narrows outwards so that processing and defibering the fibre matter in the refiner can be done as desired. Because the material to be defibered always contains a significant amount of moisture, a great deal of vapour is generated during defibering, which affects the operation and behaviour of a disc refiner in many ways.
For controlling the operation of the refiner, it is necessary to be able to move the refining surfaces to a suitable distance from each other. For this purpose, a loader is typically connected to act on one refiner disc so as to push the refiner disc towards the second refiner disc or to pull it away from it depending on the internal pressure conditions in the refiner. The force caused by the pressure between the refining surfaces of the refiner can in a normal refiner be negative or positive depending on for instance vapour pressure, flows of the refining material affected by the geometry of the refining surfaces, counter-pressure of the refining chamber and many other factors. Thus, when the gap between the refining surfaces in some applications is quite small, there is a danger that the refining surfaces touch each other and cause extra wear and possibly even bigger damage. In special situations, in which a low loading force is used and the pressure situation between the discs may change from positive to negative, this risk is quite high.